Typically thorax image data acquisition occurs in an inhaled or in an exhaled state. The patients hold their breath for the duration of the thorax scan, which prevents breathing motion artifacts, but still the heart is beating. The heart motion results in inconsistencies within the set of projections from which a CT scan is reconstructed. This results in motion artifacts such as tissue dislocation, blurring, and spurious edges. A method of imaging moving tissue is described in US 2005/0069081 entitled “Method for Tracking Motion Phase of an Object for Correcting Organ Motion Artifacts in X-Ray CT”. This document describes a method of tracking the motion phase of an object such as a heart. Within the diastole phase of a cardiac cycle there is less motion of the heart than in the rest of the cardiac cycle. The described method of tracking the motion phase of an object identifies the projection data acquired during the diastolic phase of the cardiac cycle. Once these projections have been identified, they are used in the tomographic image reconstruction process, producing images substantially free of cardiac motion artifacts. If this method is to be used, however, the data acquisition time must extend over a plurality of diastolic phases, i.e. over a plurality of cardiac cycles, thus requiring an extended data acquisition time. Holding the breath during the extended data acquisition time may be inconvenient or sometimes impossible for a patient.